Economics, 3rd Edition

perfect complements

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Perfect Substitutes and Perfect Complements

When two goods are easily substitutable, such as 50  cent and 10  cent coins, the indifference curves are straight lines, as shown in 

panel (a). When two goods are strongly complementary, such as left shoes and right shoes, the indifference curves are right angles, 

as shown in panel (b).

Left

shoes

Right shoes

(b) Perfect complements

7

5

0

5

7

I

2

I

1

10 cent

15

10

50 cent

(a) Perfect substitutes

2

3

1

0

5

I

1

I

2

I

3

P

f

t

S

b tit t

FIGURE 5.10

perfect complements 

two goods with right-angle indifference curves

SELF TEST 

Why is the amount of a good a person is consuming at a given point in time an important factor 

in determining the marginal rate of substitution? What happens to the total and marginal utility of two goods, 

x and y, if a consumer has a large quantity of good x but hardly any of good y and then opts to consume more 

of good 

x ?

In the real world, of course, most goods are neither perfect substitutes (like coins of different denom-

inations) nor perfect complements (like right shoes and left shoes). More typically, the indifference curves 

are bowed inward, but not so bowed as to become right angles.